There is known in the prior art, in particular from WO Patent No. 2004/081695, and corresponding U.S. equivalent publication U.S. Patent Application Publication No. US 2007/0069604 A1, both of which are incorporated herein by reference, a micro-motor made in a silicon wafer. In that document, the motor is made by etching a silicon layer. It includes a toothed drive wheel and actuating fingers, which cooperate with the teeth of the wheel to cause it to rotate. Each actuating finger is secured in movement to a mobile comb, which moves relative to a fixed comb as a function of a voltage V.
Interdigited comb actuators of this type are often employed for low power mechanical actuation, such as optical switches, and they are not normally operated to achieve maximum efficiency, but are rather optimised for a minimum actuation time. In such case, the actuator is powered with a lot of energy, the excess energy being eliminated at the end of the actuation phase. This is far from being optimal in terms of energy saving.
When this type of actuator is integrated in a timepiece, which is generally powered by a battery of relatively limited capacity, the power consumption of the timepiece movement motor is critical and must therefore be reduced as far as possible.
EP Patent No. 1,793,208 discloses a servo-mechanism that includes an electrostatic actuator comprising a first fixed electrode structure and a second electrode structure, which is mobile between a first and second position, the electrode structures having a variable electrostatic capacitance depending upon the position of the mobile electrode structure. The servo-mechanism also includes a low voltage supply source, a battery for example, and a voltage booster circuit, powered by the battery. The voltage booster circuit is connected to produce a voltage between the two electrode structures, so as to generate an electrostatic force between the fixed electrode structure and the mobile electrode structure.
This prior art document teaches that it is possible to optimise the power consumption of the servo-mechanism by applying pulses with the “right voltage” and for the “right duration” between the electrode structures. In order to do this, the servo-mechanism also has regulating means, which are for supplying control pulses of variable duration to the voltage booster circuit. The regulating means supply a longer control pulse if the displacement amplitude of the mobile structure is less than a threshold value, and supply a shorter control pulse if the displacement amplitude of the mobile structure is higher than the threshold value.
According to this document, the voltage booster circuit is a step-up charger type inductive circuit. There is not therefore a voltage generator for generating a determined potential difference. In this type of circuit, the duration of the control pulses influences both the duration of the pulses supplied by the voltage booster circuit and the behaviour of the voltage. In particular, it is not possible to vary the output voltage independently of the duration of the output pulses.